Hydraulic cylinder for providing reciprocation of a hydraulic jack

ABSTRACT

This invention relates to a device for feeding a double-acting jack. A rotary engine is coupled to the jack-feeding distributor in order to place the chamber of this jack in communication either with the delivery of a pump or with a fluid reservoir. One application of this feed is the production of a vibration generator.

I United States Patent [191 Martin 1 Nov. 4, 1975 HYDRAULIC CYLINDER FOR PROVIDING RECIPROCATION OF A HYDRAULIC JACK [75] Inventor: J. Louis E. Martin, Senlis, France [73] Assignee: Poclain, Le Plessie Belleville,

France 22 Filed: Feb. 20, 1973 21 Appl. No.2 333,555

301' Foreign Application Priority Data Feb. 24, 1972 France 72.06315 52 A US. Cl. 91/417; 60/369; 91/40; 91/436; 91/437; 91/441; 91/470 [51] Int. Cl. F1513 15/17 [58] Field of Search 60/369; 91/318, 321, 39, 91/40, 436, 437, 470, 420, 421, 441, 416, 417; 180/792 R [56] References Cited UNITED STATES PATENTS 2,550,723 5/1961 Ross 60/369 X 2,619,074 11/1952 Court 2,800,885 7/1957 Panissidi 2,807,141 9/1957 Strader .Q 91/40 X 2,814,182 ll/1957 Adams et a1 91/40 X 2,917,028 12/1959 Schonherr 91/40 X 2,980,079 4/1961 Joelson 91/321 3,334,547 8/1967 Grundmann 91/338 X 3,459,401 8/1969 Weaver 91/39 X 3,759,144 9/1973 lkeda 91/436 FOREIGN PATENTS OR APPLICATIONS 1,300,329 7/1969 Germany 9l/321 OTHER PUBLICATIONS Gonnan, Why Does A Pump Pump, Popular Science, August 1962, pp. 107-110 & 179.

Primary Examiner-C. J. Husar Assistant ExaminerEdward Look Attorney, Agent, or Firm-Mason, Fenwick & Lawrence ABSTRACT This invention relates to a device for feeding a doubleacting jack. A rotary engine is coupled to the jackfeeding distributor in order to place the chamber of this jack in communication either with the delivery of a pump or with -a fluid reservoir. One application of this feed is the production of a vibration generator.

1 Claim, 9 Drawing Figures U.S. Patent N0v.4, 1975 Sheet20f5 3,916,768

US. Patent Nov. 4, 1975 Sheet 3 of5 3,916,768

US. Patent Nov. 4, 1975 Sheet40f5 I 3,916,768

U.S. Patent Nov. 4, 1975 SheetS )fS 3,916,768

HYDRAULIC CYLINDER FOR PROVIDING RECIPROCATION OF A HYDRAULIC JACK BACKGROUND OF THE INVENTION The subject invention is in the field of vibration generating equipment and is specifically directed to hydraulic fluid control circuitry associated with a fluid actuating jack comprising a hydraulic cylinder and piston assembly which is operated to provide cyclic reciprocation of the piston member and rod assembly with respect to the cylinder.

The employment of hydraulic jack assemblies for positioning of machine elements or the like is well-known and has been used for many years. Such usage is primarily for the purpose of positioning a member in a fixed position in which it remains for at least a short time period. Other fluid actuated hydraulic jack members have been employed for providing vibration by the cyclic reciprocation of the piston in the hydraulic cylinder. Such devices have been of particular use in material fatigue testing operations.

The prior known hydraulic piston and cylinder assemblies and control circuitry for providing vibrational forces have frequently been complex in construction and have consequently been expensive to fabricate and maintain. Additionally, the high degree of reliability required for material testing procedures has not been uniformly achieved by the prior art devices.

SUMMARY OF THE INVENTION Therefore, it is the primary object of this invention to provide a new and improved hydraulic jack and control assembly capable of providing reliable cyclic reciprocation of the piston rod of the jack assembly.

Achievement of the object of this invention is enabled by the preferred embodiment through the employment of a jack comprising a hydraulic cylinder and piston assembly with automatic reciprocation of the piston within the cylinder being effected by the inventive hydraulic control circuitry. In all cases, the piston is acted upon by a biassing force applied to the piston rod side of the cylinder tending to move the piston in a direction moving the rod into the confines of the cylinder. The cyclic application of hydraulic fluid at a pressure sufficiently high to overcome the aforementioned biassing force is provided in the cylinder on the side of the piston opposite the piston rod so as to achieve a reciprocation of the piston under the control of the application of the pressurized hydraulic fluid. The system includes:

a fluid reservoir a source of pressurised fluid,

a distributor valve spool member movable between two end positions first, second and third conduits connecting said distributor to the working chamber, fluid reservoir and source of pressurised fluid repsectively, whilst, in its first end position, the distributor places the first and third conduits in communication and blocks the second conduit; in its second end position, the distributor places the first and second conduits in communication and blocks the third conduit.

a fluid-actuated rotary engine coupled to said distributor,

a forth conduit connecting the inlet of said rotary engine to the source of pressurised fluid,

a restriction disposed on said fourth conduit, and

is between the two end positions in which the first conduit is blocked by the distributor a calibrated valve is disposed on the fifth conduit between the fluid reservoir and the delivery of the rotary engine. Moreover, a sixth conduit connects the first conduit to the part of the fifth conduit between said rotary engine and calibrated valve, whilst a nonreturn valve is disposed on said sixth conduit, allowing the fluid to pass from the fifth conduit towards the first conduit.

A seventh conduit advantageously connects said first conduit to the source of pressurized fluid, whilst another nonreturn valve is disposed on said seventh conduit allowing the fluid to pass from the first conduit towards the source.

The seventh conduit is preferably connected to the source, by being connected to the part of the fourth conduit included between said source and the restriction.

In another embodiment, the subject invention additionally includes an accumulator conduit connecting a pressurized fluid accumulator to the source of pressurized fluid comprising the output from a power driven pump. A second distributor valve is disposed in the accumulator conduit for enabling communication of pressurized fluid in the accumulator with the source of pressurized fluid in a first position but being movable to a second position to block the accumulator conduit and prevent communication between the pressure accumulator and the source of pressurized fluid. In this embodiment, the restriction in the fourth conduit is an adjustable valve member which is adjustable for providing a desired flow rate to the rotary engine driving the distributor valve so as to enable adjustment of the frequency of reciprocation of the piston rod assembly. Additionally, the second conduit is connected to the fluid reservoir through a connection to the fifth conduit. In all of the embodiments, the distributor valving means can either be a reciprocating valve or a rotary valve member.

In another embodiment, the jack is a single-action jack in that it only has one working chamber into which the pressurized hydraulic fluid is injected. An elastic member such as a spring or the like is interposed in the cylinder on the side of the piston opposite the side in which pressurized fluid is provided to oppose the movement of the piston caused by the pressurized fluid so that a lowering of "the pressure in the working chamber below a preset level will permit the elastic element to move the piston to an extreme end position.

According to another embodiment, the jack is a double-acting jack. A piston rod is coupled to the piston of this jack, this piston defining, inside the cylinder, a first chamber of larger volume having one portion defined by the face of the piston from which the piston rod does not extend and a second chamber of smaller volume having one portion defined by the face of the piston from which the piston rod extends. The larger chamber constitutes the working chamber and the smaller chamber is connected by a ninth conduit to the source of pressurized fluid for moving or returning the piston to an end position when the source of pressurized fluid is not connected to the larger chamber.

The source of pressurised fluid with which the third conduit and the source of pressurised fluid with which the ninth conduit is connected are advantageously ment of the invention;

FIG. 8 is aschematic diagram of another embodi-j 3 comprise a single pump.

However, the variant is sometimes preferred in which the source of pressurised fluid with which the third conduit is connected is a pressurised fluid accumulator, whilst the sourceof pressurised fluid with which the ninth conduit is connected is a pump.

Finally, according to a third embodiment, the jack is a double-acting jack, a piston rod is coupledto the piston of this jack and this piston defines, inside the cylinder, a large chamber and a small chamber, the piston rod being at least partly contained in said small cham-' her. The small chamber in this embodiment constitutes v the working chamber, whilst the large chamber is connected to a source of pressurised fluid by a tenth conduit and the pressurised fluid that may be contained'in said large chamber constitutes the return device. An elastic element is interposed between the cylinder and the piston and its effect is antagonistic to thatof fluid contained in the largechamber.

The source of pressurised fluid with which the third conduit is connected and the pressurised fluid source the with which the tenth conduit is connected advanta; geously by a single pump.

. BRIEFDESCRIPTIQN or f FIG.,1 is a schematic illustration of a preferredem- DRAWINGS bodiment of the invention;

the exhaust of said engine to reservoir 9 respectively. A

restriction 21 mounted in the conduit 19 serves to control theamount of fluid flowing through conduit 19.

Additionally, it shouldbe noted that the distributor 13 can assume a third orinterrnediate position which is not shown in the drawings but in which the conduit 14 is blocked.

In a first embodimenna calibrated valve 22 is disposed'o'n the conduit 20. Conduit 14 is connected to I the part of conduit included between the exhaust of i 'called by a conduit 23. A so-called first nonretum valve the-rotary'engine 18 and the calibrated valve 22 so- 24 is mounted in conduit '23 and allows the fluid to pass 1 from conduit 20 towards conduit 14.

FIG. 2 is a simplified schematic diagram of the embodiment of FIG. 1 illustrating the distributor in an end position causing movement of the piston to the left;

FIG. 3 is a simplified schematic viewof the embodiment of FIG. 1 illustrating the distributor in its second end position in which the piston would move to the 7 right as illustrated therein;

FIG. 4 is a perspective view with parts removed of rotary distributor means operable in the FIGS. 1-3; I

FIG. 5 is a schematic illustration of another embodiment of the invention;

FIG. 6 is an enlarged sectional view of an adjustable" .FIG. 5;

FIG. 7.is a schematic diagram of another embodiment of the ,inventiomandv ment of the invention.

.Referring now tothe drawings, FIG. 1 shows a device which comprises a double-acting jack 1 provided with a piston 2 and a piston rod 3. The chamber 4 inwhich the piston rod 3 is disposed, has a so-called small useful section, smallerthan the so-called large useful section, of the chamber 5 opposite thereto. The foregoing relationship is true because of the fact that the face of the piston 2 defining part of the chamber 4 is smallerthan the face of the piston 2 defining part of the chamber 5. A body 6 is fixed to the rod 3 by means of a housing 7 with which it is provided and a pin 8 or like means.

The device further comprises a fluid reservoir 9 and a pump 10, the inlet of which is connected to the reser- In a second embodiment, a conduit 25 connects conduit l4'to conduit 12. A so-called second nonretum valve 26 is disposed on this conduit 25 and allows the fluid to pass from conduit 14 towards conduit 12.

FIG. 2 shows a simplified diagram of FIG. 1. It consists in connecting conduit 25 to conduit 12, not directly, but connecting said conduit 25 to the part of conduit 19 included between" conduit 12 and the restriction 21. Similarly; conduit 15 is connected to the fluid reservoir 9 by means of conduit 20, with which it embodiment of voir 9 by a conduit 11. A conduit 12 connects the delivample of which is shown in FIG. 4, with a rotary engine 18. The distributor 13 is connected by conduit 14 to is connected.

FIG. 4 shows a rotary type distributor 13 known per se. It is constituted by a cylindrical body 27, inside which a rotor 28 is mounted to rotate. The rotor 28 is provided with hollowed out sectors 29, separated by solid sectors 30 and has a shaft 3l which is provided with a connecting flange 32. The engine 18 also has a driven shaft 33 provided with flange 34, the flanges 32 and 34 being fixed together by bolts 35. The conduits 14, 15, and 16 are in factdivided into two, viz. 14a, 14b, 15a, 15b, 16a, l6b,where they are connected with the rotary distributor 13. It is noted that the distributor 13 shown in FIG. 4 operates to provide communication between the various conduits and blockage of the various conduits exactly as described with respect to FIG. 1'. Finally, it will be noted that the embodiment of FIG. 4 is certainly advantageous, but is not the onlyone that may be chosen. In particular, it will be understood that a rectilinearly moving slide distributor as in FIG. 1 coupled with a tierod-crank system or with an eccentric, may fulfill the function devolving upon the rotary distributor 13.

FIG. 5 illustrates an embodiment of the inventionsimilar to FIG. 1 with the exception that the embodiment of FIG. 5 employs an adjustable flow restricting means 21a and also employs a pressurized fluid accumulator device 36. A second distributor 38 which merely constitutes an on-off valve is mounted in an accumulator conduit-defined by conduit portions 37a and 371 as clearly shown in FIG. 5. The second distributor 3 8 is movable between one or two positions with the first position being as illustrated in FIG. 5 in which the pressure accumulator is in communication with conduit 12. The second positionof the distributor 38 is essentially a blockage position in which the pressure accumulator is isolated.

FIG. 6 shows an embodiment, known per se, of the adjustable restriction 21a. The conduit 19 is provided with a boss 39 which restricts the section of passage thereof. Furthermore, a piece 40 is mounted to slide on the conduit 19, in which it is introduced through an opening 41 with interpostion of gaskets 42..The piece 40 is partly disposedopposite the section of passage 8 of the conduit 19, at thelevel of the boss 39, and restricts this section to a values lower than valu'e S. A support 43 is welded to the conduit 19 and is provided with a threading complementary to that of a threaded screw 44 inserted therein. The screw 44 isprovided with a wheel 45 for controlling its rotation. One of the ends of the screw 44 is inserted into an aperture 46 with which the piece 40 is provided and is integral in translation with said piece 40 by means of two pairs of nuts-counternuts 47.

With reference to FIG. 7, the device is the same as in FIG. 1, except that the jack 101 shown is a singleaction jack. The piston 102 mounted to slide in the cylinder of the jack 101 thus defines with said cylinder :1 single working chamber 105. The piston rod 103 is fixed to the piston 102 and is partly contained in chamber 104 opposite chamberl05. It is noted that a spring 106 is disposed in the chamber 104 between the piston 102 and cylinder and has an antagonistic effect to that of the fluid that may be contained in the chamber 105. Therefore, it will be seen that the operation of the spring 106 opposes movement of the piston 2 to the left as viewed in FIG. 7 so as to oppose the action of the pressurized fluid in chamber 105. An opening 107 made in the wall of the cylinder connects the chamber 104 to atmosphere. Finally, the delivery conduit of the pump is shown at 112 and is connected only to conduits 16, 19 and 25, but on the other hand is not connected to the small chamber 104 as the conduit 12 of FIG. 1 is to chamber 4.

The arrangement shown in FIG. 8 shows a variant of the actual feed of pressurised fluid. A pump 210 is connected to the reservoir 9 by its inlet conduit 21 l and to the chamber 4 by its delivery conduit 213. Furthermore, a conduit 237 is connected to a pressurised fluid accumulator 236. The conduits 16 and 25 are themselves connected to conduit 237, a conduit 219 connecting the restriction 21a to said conduit 16. A conduit 215 connects the conduit213 toreservoir 9, a calibrated discharge valve 214 being disposed on said conduit 215.

Finally, FIG. 9 shows an arrangement which is similar to the one shown in FIG. 1. However, it will be noted that the delivery conduit of the pump 10, referenced at 312 is no longer connected to chamber 4 but to large chamber 5. In the same way, the third conduit connected to the distributor 13, other than conduits and 16, is referenced at 314 and connects said distributor to small chamber 4. The conduits 16, 19 and 25 are connected to conduit 312, as they were to conduit 12 of FIG. 1, in the same way as the conduits 23 and 25 are connected to conduit 314, as they were beforehand to conduit 14 of FIG. 1. A spring 301 is interposed between piston 2 and the cylinder of jack 5 and has an antagonistic effect to that of the fluid that may be contained in the chamber 5. I

The advantages offered by the device according to the invention will be more readily understood 'upon reading a statement of the operation thereof.

In the first place, a reciprocating movement is gener- 'ated.

It is assumed, that the distributor is disposed according to what is shown in FIG. 2. The pressurised fluid delivered by the pump 10 is directed on the one hand through conduit 12 and on the other hand through conduits 12, 16 and 14 towards chambers 4 and 5, respectively, of jack 1. The effect of this fluid on the piston 2 tends to repel this latter in the direction of arrow F. Furthermore, the pressurised fluid reaches the inlet of the engine 18 through conduits 12 and 19, through the restriction 21. Consequently, the engine ,18 is rotated and actuates the distributor 13 in order to dispose it from its first to its second position. I

Furthermore, it will be noted that the overall inertia of the body 6 and the rod and piston 3 of the piston 2 which are coupled thereto opposes their movement in the direction of arrow F. The pressure of the fluid therefore increases in chamber 4 ofthe jack 1 and consequently in conduit 12. The flow of fluid through the restriction 21 therefore increases too, this having for its effect to increase the speed of the engine 18 and to accelerate the change of position of the distributor 13. It is therefore realised that the speed of the change of position of this distributor 13 is a function of the value of the overall inertia of the body 6 coupled to the piston rod 3, or that the frequency of the movement of the body 6 is a function of the value of the overall inertia.

Finally, the distributor 13 reaches its position as shown in FIG. 3. Under the effect, this time, of the pressurised fluid which acts only on one face of the pistona2, which defines chamber 4, the piston 2 tends to return, and returns, in the opposite direction to arrow F. The fluid contained in chamber 5 returns to reservoir 9 via conduits 14, 15 and 20. The rotary engine 18 continues to drive the distributor 13 until it is-put back into its position of FIG. 2. A new cycle of operations may then commence.

It is understood that, similarly to what was stated above, the spring 106 of the device of FIG. 7 has an effect of returning the piston 102 into position, similar to the one produced by the action of the pressurised fluid, which was contained in chamber 4 of the device of FIG. 1 on piston 2. It is therefore not necessary to describe the operation of the device of FIG. 7 which in fact is similar to that of the device of FIG. 1 or of the variant embodiment of FIG. 2 which has already been given.

In the same way, it is understood that the assembly of the two distinct sources of fluid, the pump 210 and the pressurised fluid accumulator 236 in the device of FIG. 8, is an equivalent to the single pump 10 of FIG. 1. There again, the description of the operation of the device of the variant of FIG. 1, shown in FIG. 2 which has already been given, is also good for the device of FIG. 8. It will merely be noted that the pressure in the conduit 213 is limited to the maximum value corresponding to the calibration of the discharge valve 214.

Finally, the functioning of the deviceshown in FIG. 9 is also similar to that of FIG. 1 or its variant of FIG. 2, described hereinbefore. In fact, the only differences are the reversal of the connections of conduits 312 and 314 to chamber 5 and 4 with respect to the connections of conduits 12 and 14 to chambers 4 and 5, respectively.

7 effect of spring 301 so that the operations of the devices of FIGS. 1 or 2 and 9 are clearly similar.

The role of the first and second nonreturn valves 24 and 26 will now be examined.

.Conceming the nonreturn valve 24, it will be specified that its role is apparent when the distributor 13 passes to its intermediate position, after it has left its first position of FIG. 2. At this moment, under the effect of its inertia, the body 6 follows its course somewhat in the direction of arrow F. However, the conduit l4is blocked at the level of the distributor 13. If the conduit 23 had not been provided, the movement of the piston would cause an at least partial vacuum in chamber 5, and would therefore cause the cavitation of the fluid in said chamber 5, which would be a drawback.

In the embodiment of FIG. 2 for example, the fluid of the part of the conduit 20 included between the engine 18 and the calibrated valve 22 is maintained by said calibrated valve 22 under a certain pressure. Via conduit 23, through the non return valve 24, this fluid reaches conduit 14 and chamber 5, whilst filling in the provisory lack of fluid. The cavitation is therefore avoided. Moreover, when the distributor 13 is diposed in its first position, the pressurised fluid does not return from conduit 14 towards reservoir 9 via conduit 23, since the non return valve 24 prevents passage thereof towards said reservoir. I

There remains the role of the non return valve 26 whose effect acts at the moment when the distributor 13 .passes from its second to its first position, this distributor then being at its intermediate-position in which it blocks the conduit 14. The pressurised fluid acts in the chamber 4 and pushes the piston 2 in the direction opposite the arrow F. The fluid of chamber 5, if the conduit 25 had not been provided, would remain fixed,

in said chamber which would risk being damaged by an excess pressure. However, there is no question of this, since the fluid of the chamber may in fact escape through conduit 25 through the non return valve 26. Of course, when the distributor l3is disposed in its second position (FIG. 3), the pressurised fluid contained in the conduit 12 cannot escape through the nonretum valve 26, which prevents passage thereof. Consequently, it may act on the engine 18 through the restriction 21 and on the piston 2 in the chamber 4, which is the sought after result.

Obviously, no emphasis will be placed on the simplicity of the diagram of FIG. 2 with respect to that of FIG.

' 1, nor on the simplicity of production brought about by thev adoption of a rotary distributor such as that of FIG. 4.

The device of FIG. 5, for a given adjustment of the value of the restriction 21a, and when the distributor 38 is in its second position, in which the accumulator 36 is isolated at its level, obviously possesses the same functioning as the device shown in FIG. 1 from which it is in no way different.

The fact of disposing the distributor 38 in its first position has for its consequence to place the accumulator 36 in communication with conduit 12. The pressure in said conduit 12 is thus at a value which is regularised with respect to the value that it had previously, when the distributor 36 was in its second position. The points of maximum pressure are lowered and it is then possible to actuate the body 6, of which the mass remains unchanged, with a clearance of amplitude different 8 from that of the clearance corresponding to the first mode of actuation.

If the value of the section s of the restriction 21a is now modified, the flow of fluid through said section 5 is modified, this causing a correlative variation in the speed of rotation of the motor 18. The frequency of the changes in position of the distributor 13 is therefore also modified, this enabling the frequency of the oscillations of the clearance of the body 6 to be varied. It will be noted in particular that by reducing the section s to a zero value, the clearance of the body 6 may even be stopped. I

The invention may be applied particularly to the production of vibration generators or further to the design of mechanisms for actuating vibrating teeth of the bucket of a public works machine to the mechanism for actuating a pneumatic drill.

What is claimed is:

1. A hydraulic fluid circuit system for providing pressurized control fluid to provide cyclic operation of a double-action hydraulic jack, said system including a hydraulic jack cylinder, a piston mounted in said hydraulic cylinder for reciprocation, said piston having a first face defining one side of a first chamber in said hydraulic chamber and a second face on an opposite side, said second face being smaller than said first face and defining one side of a second chamber in said hydraulic cylinder, a piston rod extending from said second face and being at least partly contained within said second chamber, said hydraulic cylinder, piston and piston rod comprising said hydraulic jack, a fluid reservoir, a distributor valve means positionable in first and second positions, a first conduit connecting said distributor valve means to one of said chambers, a second conduit connecting said distributor valve means to said fluid reservoir, a third conduit connecting said distributor valve means to a fluid accumulator which comprises a source of pressurized fluid, a conduit connecting said second chamber to the output of a pump to provide movement of said piston toward said first chamber when said first chamber is below a predetermined pressure level, said distributor valve means including means defining a passageway providing communication between said first conduit and said third conduit while simultaneously blocking said second conduit when said distributor valve means is in its said first position, said distributor valve means further including means directly communicating said first conduit with said second conduit while simultaneously blocking said third conduit when said distributor valve means is in its second position, a fluid-actuated rotary engine having an inlet and an exhaust, coupling means coupling said rotary engine to said distributor valve means formoving said distributor valve means between said first position and said second position, a fourth conduit connecting the inlet of said rotary engine to the fluid accumulator, a flow restriction means mounted in said fourth conduit, a fifth conduit connecting the exhaust of said rotary engine to said fluid reservoir, calibrated pressure responsive valve means mounted in said fifth conduit between said fluid reservoir and said exhaust of the rotary engine, a sixth conduit connecting the first conduit to a portion of the fifth conduit between said rotary engine exhaust and said calibrated valve means and a non-return valve mounted in said sixth conduit for allowing hydraulic fluid to pass from said fifth conduit toward said first conduit. 

1. A hydraulic fluid circuit system for providing pressurized control fluid to provide cyclic operation of a double-action hydraulic jack, said system including a hydraulic jack cylinder, a piston mounted in said hydraulic cylinder for reciprocation, said piston having a first face defining one side of a first chamber in said hydraulic chamber and a second face on an opposite side, said second face being smaller than said first face and defining one side of a second chamber in said hydraulic cylinder, a piston rod extending from said second face and being at least partly contained within said second chamber, said hydraulic cylinder, piston and piston rod comprising said hydraulic jack, a fluid reservoir, a distributor valve means positionable in first and second positions, a first conduit connecting said distributor valve means to one of said chambers, a second conduit connecting said distributor valve means to said fluid reservoir, a third conduit connecting said distributor valve means to a fluid accumulator which comprises a source of pressurized fluid, a conduit connecting said second chamber to the output of a pump to provide movement of said piston toward said first chamber when said first chamber is below a predetermined pressure level, said distributor valve means including means defining a passageway providing communication between said first conduit and said third conduit while simultaneously blocking said second conduit when said distributor valve means is in its said first position, said distributor valve means further including means directly communicating said first conduit with said second conduit while simultaneously blocking said third conduit when said diStributor valve means is in its second position, a fluid-actuated rotary engine having an inlet and an exhaust, coupling means coupling said rotary engine to said distributor valve means for moving said distributor valve means between said first position and said second position, a fourth conduit connecting the inlet of said rotary engine to the fluid accumulator, a flow restriction means mounted in said fourth conduit, a fifth conduit connecting the exhaust of said rotary engine to said fluid reservoir, calibrated pressure responsive valve means mounted in said fifth conduit between said fluid reservoir and said exhaust of the rotary engine, a sixth conduit connecting the first conduit to a portion of the fifth conduit between said rotary engine exhaust and said calibrated valve means and a non-return valve mounted in said sixth conduit for allowing hydraulic fluid to pass from said fifth conduit toward said first conduit. 